We have shown through immunofluorescence and confocal microscopy that centrosomes of high grade breast tumors are larger and more numerous than centrosomes in normal breast tissue. Additionally, the centrosomes in breast tumor cells are hyperphosphorylated and are capable of nucleating a larger number of microtubules in vitro as compared to centrosomes from normal breast cells. We describe these centrosomes as "hypertrophic". Attempts to characterize hypertrophic centrosomes at an ultrastructural level using CTEM have been difficult. This is due, at least in part, to the small volume of the centrosomes relative to the total volume of the cells; the centrosomes are few and far between in 90 nm thick sections through cells that are 6-10 um thick. In addition, the hypertrophic centrosomes we have observed with CTEM have been especially electron opaque, making it difficult to resolve microtubules and other structural components of the centrosomes. Initial results using 0.5 and 1 micron thick sections have proven not only to greatly facilitate the locating of hypertropic centrosomes, but have also produced 3-dimensional images with heretofore unseen structural detail. Several tomographic reconstructions of centrosomes from cancerous cells were performed when Dr. Lingle visited the laboratory last summer. Additional reconstructions are planned in the near future.